#define LED 13
#define OFF_TIME 3500
#define ON_TIME 500 

bool ledState = LOW;
unsigned int previousMillis = 0;
unsigned int myTime;

void setup() {
  Serial.begin(9600);
  pinMode(13, OUTPUT);
}

void loop() {
  unsigned int currentMillis = millis();
  if ((currentMillis - previousMillis >= OFF_TIME) && (ledState == LOW)) {
    previousMillis = currentMillis;
    ledState = HIGH;
    digitalWrite(LED, HIGH);
  }

  if((currentMillis - previousMillis >= ON_TIME) && (ledState == HIGH)){
    previousMillis = currentMillis;
    digitalWrite(LED, LOW);
    ledState = LOW;
  }
}